Cockpit window blowouts do happen and the door has to accommodate this. Also it would be nice if the cabin crew can gain entry to help keep pilot x from going all the way out the window so that pilot y can get on with the job of flying the airplane.
With a strengthened door, it will be important to ensure that two crew members are always present in case of incapacitation or decompression. It won't do for the captain to have a heart attack while the co-pilot is visiting the lav and can't get back in.